Modern refrigerator cabinets may comprise one compartment or several compartments kept at different temperatures. For household applications and also for mobile applications, such as in mobile homes and caravans, the refrigerator may comprise a freezer compartment kept at approx. −18° C. and a fridge compartment kept at approx. +5° C. The refrigerator comprises a refrigerator apparatus including a condenser and an evaporator. Compressor refrigerators further comprise a compressor, whereas absorption refrigerators instead further comprise a boiler and an absorber. The evaporator comprises an evaporator tube for conducting a cooling medium. The evaporator tube is arranged so that it passes inside the compartment or compartments, which is or are to be cooled by the refrigerator apparatus. For enhancing the heat transfer from the air in the compartments to the cooling medium, a heat exchanger is arranged in heat conducting contact with a portion of the evaporator tube arranged in the respective compartment. The main function of the heat exchanger generally is to enlarge the surface area of the heat conducting material, which is in contact with the air to be cooled and the cooling medium in the evaporator tube. For this purpose the heat exchanger typically comprises a plurality of fins, which are arranged in heat conducting contact with the evaporator tube.
During normal operation of the refrigerator cabinet, humid air enters into the compartments e.g. when the cabinet doors are opened. As the humidity condenses on the cold surfaces inside the compartments, frost is created on these cold surfaces. Such development of frost is particularly severe on the coldest surfaces, i.e. on the evaporator tube and the heat exchanger in the freezer compartment. The formation of frost on the heat exchanger deteriorates the heat transfer from the air to the cooling medium and thereby lowers the cooling power of the compartment. If the refrigerator apparatus is not dimensioned to compensate for such loss in heat transfer, the temperature in the compartment rises, while jeopardizing the condition of the foodstuff stored in the compartment or the maximum possible storage time. In order to solve this problems, modern refrigerators may comprise means for defrosting the heat exchanger at regular intervals. In such case, the defrosting means is normally applied to the heat exchanger in the freezer, but it may also be applied in the fridge.
U.S. Pat. No. 4,432,211 describes a defrosting apparatus for defrosting the heat exchanger or cooler of a refrigerator. The heat exchanger comprises a plurality of rectangular fins, which are arranged in heat conducting contact with the evaporator tube. The evaporator tube is formed as a coil, comprising two parallel coil portions, each portion comprising a number of straight horizontal tube sections arranged one above the other and connected one to the other by vertically oriented U-shaped tube bends. The two coil portions are connected to each other by a horizontally oriented U-shaped tube bend. The evaporator coil thus comprises two coil portions, generally extending in respective vertical extension planes arranged next to each other. The rectangular fins extend parallel to each other in respective vertical extension planes, which are perpendicular to the vertical extension planes of the coil portions. The straight tube sections of both coil portions are arranged through openings arranged in a mid portion, between the edges of each fin. The evaporator tube makes contact with the fins at each opening for conducting heat from the fin to the cooling medium inside the tube. This arrangement allows for air to be cooled to pass between the fins and thereby to contact the surfaces of the fins and the evaporator tube sections arranged between the fins, whereby heat may be conducted from the air to the cooling medium.
The U.S. Pat. No. 4,432,211 arrangement further comprises means for defrosting the fins and the evaporator coil. This defrosting means consists of a heater element, which is attached to the vertical edges of the fins, either on one or on both opposite sides of the fins.
WO 03/008880 A1 describes a similar arrangement where the evaporator coil is arranged perpendicular to the fins and through openings arranged in the fins. A heating element in the form of a resistive sheet is arranged in contact with the edges of the fins, at one side of the evaporator coil. For enhancing the heat transfer from the resistive film to the fins, the edge portion of the fins may be L-shaped such that the contact area between the film and the fins is enlarged. Both the above described arrangements functions in generally the same manner. The heating element is activated at regular intervals. Thereby, heat is generated and conducted from the heating element to the fins and further to the evaporator tube. The so achieved heating of the fins and the evaporator tube melts any frost, which is formed on these members. Control means may be provided for turning off the heating element when all frost has been melted.
Even though the above-described defrosting arrangements may achieve full defrosting of the heat exchanger, they are also impaired with some disadvantages. A major disadvantage concerns the arrangement of the heating element in relation to the fins and the evaporator tube. In both the prior art arrangements, the evaporator tube is arranged through openings arranged in mid portions, between the edges, of the fins. The heating element on the other hand, is arranged in contact with one edge of the fins. This means that there will always be a portion of each fin which is arranged on the opposite side of the evaporator tube as seen from that edge of the fin, which is in contact with the heating element. Expressed differently, a portion of each fin is located at a greater distance from the heating element than the opening surrounding the evaporator tube.
As a consequence, defrosting heat generated by the heating element always has to be transferred past the opening and the evaporator tube in each fin, before it reaches that portion of the fin, which is arranged on the remote side of the opening, for defrosting this remote portion. Therefore a substantial amount of defrosting heat is transferred to and absorbed by the cooling fluid in the evaporator tube, instead of being used for defrosting the remote portion of the fins.
This arrangement is most unfavorable for several reasons. Firstly, the time needed for defrosting the entire heat exchanger is prolonged, since a substantial part of the generated heat is lost and not used for defrosting. For the same reason the total energy consumption of the heating element is increased. Secondly and even more important, especially at absorption refrigerators, the cooling power of the entire refrigerator cabinet is decreased since the temperature of the cooling medium in the evaporator tube rises when the medium absorbs additional heat from the defrosting heater. Due to the increase in cooling medium temperature, the ability of the evaporator to absorb heat from the air in the refrigerator compartments and thereby to maintain these compartments at the desired temperature is decreased. This is true not only for the compartment in which the defrosting heater works, but also for any compartment cooled by a portion of the entire evaporator tube, which potion is arranged downstream of the evaporator portion in contact with the defrosted heat exchanger. Normally in dual or multi compartment refrigerators, defrosting devices area applied to the heat exchanger serving the freezer compartment. Since the freezer compartment needs the coldest evaporator temperature, this compartment is cooled by the coldest, i.e. most upstream portion of the entire evaporator tube. Hence, the defrosting heat transferred from the defrosting heater to the heat exchanger in the freezer, adversely affects the cooling power of all the compartments in the refrigerator.
Even if the refrigeration apparatus and thereby the circulation of cooling medium in the evaporator tube, is stopped during defrosting, the same problems occurs. In such case, the volume of cooling medium actually present in that portion of the evaporator tube, which is arranged in proximity to the defrosted heat exchanger, will be heated to a higher temperature. After completion of the defrosting cycle and upon restart of the cooling medium circulation, this volume of cooling medium will have to be even more reduced in temperature by the refrigeration process before it can restart to absorb heat from the compartments.
A further problem associated with the above described prior art defrosting arrangements is that heat is not evenly distributed over the fins. Due to the arrangement of the evaporator tube and the fins, the resistance to heat transfer through the material of the fins will be different at different portions of the fins. This leads to significant disadvantages during defrosting as well as during normal operation of the refrigerator. During normal operation, the uneven heat distribution over the fins will lead to that frost develops more rapidly at some colder portions of the fins than on other portions. Such local development of frost might cause the air passages between the fins to be blocked, whereby defrosting is required more often than what would be needed at an even distributed development of frost.
During defrosting, the uneven distribution of frost over the fins leads to inefficient defrosting. The areas on which less frost is formed will be defrosted faster than areas with heavy frost formation. These early defrosted areas will, during the remaining defrosting cycle for defrosting the areas with heavy frost formation, transfer excessive heat from the defrosting heater to the ambient air. Thereby, a most unwanted heating of the air in the compartment is caused together with an excessive energy consumption of the heater. Further, during defrosting, the uneven heat distribution over the fins per se causes some areas of the fins to be defrosted earlier than other areas, thereby creating the same disadvantages as just mentioned.
The above-described problems connected with the prior art defrosting arrangements are particularly severe in conjunction with mobile absorption refrigeration applications. At such applications, the physical dimensions of the refrigerator cabinet, i.e. maximal allowable height of the cabinet, limit the total cooling capacity of the refrigeration apparatus. Thus, any excessive heat added directly to the evaporator or the air in the refrigerator compartments, drastically reduces the possibility to keep the compartments at temperatures as low as nowadays desired. Further more, at some mobile applications the available electrical DC energy is often limited. Thus, an excessive energy need for defrosting is most unwanted and might even lead to battery drainage causing downtime or collapse in the various electrical systems of the vehicle.